In the soil thematic strategy of the European Union Commission, a soil organic carbon content of 2% is indicated as a threshold below which a reduction in soil chemical, biological and physical fertility, and increase in erosion can be observed. Composting of organic matter ‘exogenous’ to soil (such as from municipalities, industries and agriculture sources) is recommended as an eﬀctive way to ensure the return of biomass to soil and the return of the soil organic matter losses. The composting of municipal solid wastes is seen as a strategy to divert organic waste materials from landﬁlls. A municipal source-separated solid waste compost was used in a study carried out during 2003–2006 in Southern Italy. An annual tomato-snap bean-lettuce rotation was planted on a sandy loam soil with 26 g kg-1 organic carbon under greenhouse conditions. Diﬀerent rates of compost (15–30–45 t ha- 1 on a dry weight basis) and combinations of compost at a rate of 15 t ha-1 with reduced doses of mineral N fertilizer (1 / 2 or 1 / 4 of optimal supply) were compared with an untreated control and a N, P, K fertilized control. We found that: (1) increasing compost rates produced increasing positive soil organic carbon balances. The C conversion eﬃciency was 23 and 36% with 15 and 30 t ha-1 , respectively, but declined to 28% with the highest rate of compost. Indeed, the higher the compost amounts applied, the higher the soil organic carbon losses. (2) Under tunnel-greenhouse conditions, all the fertilization strategies, except compost at a rate of 15 t ha-1, increased soil nitrate concentrations by up 100 to 400 mg kg-1 dry weight of soil, particularly in the spring-summer seasons. In the same period, nitrate contents in the untreated control reached 100 mg kg-1. (3) The average yield of marketable tomato for the four-year period was 114 t ha-1 and did not vary signiﬁcantly among treatments. No differences in snap bean yields were detected among the fertilization treatments. In lettuce cultivation, however, 30 and 45 t ha-1 of compost yielded more than other treatments. In the tunnel-greenhouse environment, a high initial content of soil organic matter resulted in high vegetable yields over all four years, even without mineral or organic fertilizer supply. However, among the various fertilization strategies, the best solution able to restore annual soil carbon mineralization was the supply of 15 t ha-1 of compost. In addition, this rate reduced the hazards linked to the high release of nitrates in soil caused by 30 and 45 t ha-1 rates of compost or mineral fertilization.
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